This might be a silly point as you are going into the RAF but have you had any flying lessons yet, this may decide whether you really want to fly for a career and really give you the motivation to get the GCSE's and A-levels?
You can probably join the air cadets to get some flying experience and over the next 9 years if you can gain a PPL that will save some time.
Does the RAF help retrain you to join civilian life? If so can they help towards commercial pilot training?!
Good luck.